Chemical Reactions. Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For.

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Presentation on theme: "Chemical Reactions. Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For."— Presentation transcript:

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Chemical Reactions

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Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For molecular substances sometimes the ratio between the atoms in a molecule is not in the simplest ratio. When the molecular and empirical formula are different then the molecular formula is a multiple of the empirical formula.

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DO NOW Which compound contains a greater % of Oxygen. Justify your answer with a calculation Li 2 O, CaO, PbO 2

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All chemical reactions… have two parts: 1.Reactants = the substances you start with 2.Products = the substances you end up with The reactants will turn into the products. Reactants → Products

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Reactants Products

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How to Describe a Reaction A reaction can be described several ways: #1. In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride. #2. In a word equation some symbols used Copper + chlorine → copper (II) chloride #3. In a chemical equation only chemical equations are used Cu + Cl 2 → CuCl 2

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Symbols used in equations the arrow → separates the reactants from the products (arrow points to products) Read as: “reacts to form” or yields or produces The plus sign + means “and” ↑ used after a product indicates a gas has been produced: H 2 ↑ ↓ used after a product indicates a solid has been produced: PbI 2 ↓

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Symbols used in equations ■ double arrow indicates a reversible reaction (more later) ■ shows that heat is supplied to the reaction ■ is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

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What is a catalyst? A substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts in your body.

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The Skeleton Equation All chemical equations are a description of the reaction. A skeleton equation uses formulas and symbols to describe a reaction but doesn’t indicate how many; this means they are NOT balanced

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ANSWERS TO WORKSHEET Crystal is a hydrate. Heating removes the water of hydration 25 Hydrated crystal is blue Anhydrous crystal is white 26 The anhydrous compound is pure. The hydrated crystal contains 50% water so it is more expensive (you are paying for water in the bottle!!!)

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Balanced Chemical Equations According to the Law of Conservation of Mass: atoms aren’t created or destroyed in a chemical reaction, they are just rearranged. All the atoms we start with in the reactants we must end up with in the products (meaning: balanced!) A balanced equation has the same number of each element on both sides of the equation.

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Rules for balancing: 1)Assemble the correct formulas for all the reactants and products, using “+” and “ → ” 2)Count the number of atoms of each type appearing on both sides 3)Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) 4)Double-Check to make sure it is balanced.

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Never change a subscript to balance an equation (You can only change coefficients) If you change the subscript (formula) you are describing a different chemical. H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula; they must go only in the front 2 NaCl is okay, but Na 2 Cl is not.

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Types of Reactions There are probably millions of reactions. We can’t remember them all, but luckily they will fall into several categories. We will learn: a) the 5 major types. We will be able to: b) predict the products. For some, we will be able to: c) predict whether or not they will happen at all. How? We recognize them by their reactants

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Synthesis Reaction AB + ABA The general equation is A + B  AB also called Combination Reactions 2 substances combine to make one compound

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Complete and balance: Ca + Cl 2 → Fe + O 2 → (assume iron (II) oxide is the product) Al + O 2 → Remember that the first step is to write the correct formulas – you can still change the subscripts at this point, but not later while balancing! Then balance by changing the coefficients only

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The general equation is : AB → A + B A reaction where a more complex molecule breaks down to form two or more simpler products Decomposition Reaction AB + ABA

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#2 - Decomposition Reactions We can predict the products if it is a binary compound (which means it is made up of only two elements) It breaks apart into the elements: H 2 O HgO mercury (II) oxide cinnabar mercury

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Single Displacement Reaction A + BCB + BAC A A reaction where an element displaces another element in a compound, producing a new compound and an element  A metal will replace a cation (metal or H)  A non-metal will replace an anion (non-metal) A + BC  AC + B

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#3 Single Displacement Reactions We can even tell whether or not a single displacement reaction will happen: More ‘active’ element replaces less active The Activity Series of Metals lists metals (and hydrogen) in order of activity. Elements higher on the list replaces those lower on the list.

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#4 - Double Replacement Reactions Have certain “driving forces”, or reasons Will only happen if one of the products: a) doesn’t dissolve in water and forms a solid (a “precipitate”), or b) is a gas that bubbles out, or c) is a molecular compound (which will usually be water).

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#5 – Combustion Reactions Combustion is a fast reaction of a substance with oxygen to make compounds called oxides. the general equation is : fuel + oxygen → oxides + energy the three things that must be present for combustion to happen are: fuel oxygen spark / heat

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SUMMARY: An equation... Describes a reaction Must be balanced in order to follow the Law of Conservation of Mass Can only be balanced by changing the coefficients. Has special symbols to indicate the physical state, if a catalyst or energy is required, etc.

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Reactions Come in 5 major types. We can tell what type they are by looking at the reactants. Single Replacement happens based on the Activity Series Double Replacement happens if one product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas.